1. Field of the Invention
The present invention relates in general to a method of allocating optimum Walsh codes for distinction between mobile station signals of a reverse link to minimize an interference between mobile stations in a code division multiple access (referred to hereinafter as CDMA) communication system.
2. Description of the Prior Art
In a CDMA communication system, generally, a reverse link from mobile stations to a base station is inferior in speech quality to a forward link from the base station to the mobile stations. In particular, a multipath fading and inter-mobile station interference on the reverse link exert an important effect on performance of the communication system.
In the forward link, all CDMA signals are distinguished from one another according to pseudo noise (referred to hereinafter as PN) codes. Also, all the CDMA signals share a pair of quadrature PN codes.
Namely, all CDMA signals, which are transmitted from one base station over CDMA channels, have the same PN code phase. These signals are distinguished from one another by mobile stations according to binary orthogonal codes based on a Walsh function.
However, there is now present no method capable of minimizing a multipath fading and inter-user interference in transmission and reception of the reverse link. Instead, there has merely been proposed a method for spreading the PN codes in the forward link.
FIG. 1 is a block diagram illustrating code spreading in a conventional reverse link.
In the conventional reverse link, as shown in FIG. 1, each mobile station signal is spread by a long PN code 1 and short PN code 2. As a result, the mobile station signals are discriminated therebetween on the basis of the long PN codes 1 allocated respectively to the corresponding mobile stations, and spread in I and Q channels by the short PN code 2.
In this manner, in the conventional reverse link, the mobile stations are equalized in interference therebetween and minimized in performance difference therebetween.
Noticeably, the degree of interference is proportional to the number of mobile stations. For this reason, the maximum number of mobile stations accommodatable by one base station is subjected to a limitation due to multipath characteristics of the mobile stations and degree of interference between the mobile stations.
The above-mentioned conventional reverse link has the following disadvantage.
Namely, in the reverse link, signals which the base station receives from the mobile stations are not in synchronization with one another under the real condition that any synchronization system is not considered. This makes it difficult to enhance a signal-to-noise ratio of the reverse link.
Therefore, the present invention has been made in view of the above problem, and it is an object of the present invention to provide a method of allocating optimum Walsh codes to a reverse link, in which a synchronization system is presented to mobile station signals in the reverse link, and the optimum Walsh codes are obtained to minimize an interference among a plurality of mobile stations and then allocated respectively to the mobile stations to spread the mobile station signals through the reverse link, so that the reverse link can significantly be enhanced in signal-to-noise ratio.
In accordance with one aspect of the present invention, there is provided a method of allocating optimum orthogonal function codes to a reverse link in a communication system which have one base station and a plurality of mobile stations, comprising the first step of detecting characteristics of reverse link channels from a desired one of the mobile stations to the base station; the second step of obtaining an orthogonal function code based on the detected characteristics of the reverse link channels; and the third step of allocating the obtained orthogonal function code to the desired mobile station.
In a feature of the present invention, in addition to conventional PN codes, optimum Walsh codes are allocated for distinction between mobile station signals to minimize an interference between mobile stations of a reverse link, thereby significantly enhancing a signal-to-noise ratio of the reverse link. The optimum Walsh codes are obtained on the basis of cross correlation.
In accordance with another aspect of the present invention, there is provided a method of allocating optimum Walsh codes to a reverse link in a mobile communication system which have a plurality of service areas, each of the service areas including one base station and a plurality of mobile stations, comprising the step of sequentially allocating Walsh codes respectively to the mobile stations in each of the service areas to distinguish signals from transmitted from the mobile stations from one another.